<template>
    <ul class="list" >
      <li class="item"
          :ref="item"
          v-for="item of letters"
          :key="item"
          @touchstart.prevent="handleTouchStart"
          @touchmove="handleTouchMove"
          @touchend="handleTouchEnd"
          @click="handleLetterClick"
      >{{item}}</li>
    </ul>
</template>

<script>
export default{
  name: 'Alphabet',
  props: {
    cities: Object
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  computed: {
    letters () {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change', e.target.innerText)
    },
    handleTouchStart (e) {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.timer) {
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style type="text/css" lang="scss" scoped>
.list{
  display: flex;
  flex-direction: column;
  justify-content: center;
  position: absolute;
  right:0;
  top:1.58rem;
  bottom:0;
  width: .4rem;
  .item{
    text-align: center;
    line-height: .4rem;
    color: $bgColor;
  }
}
</style>
